refactor(telemetry): replace unused-parameter casts with maybe_unused

setNodeId() and Counter::add() silenced an unused parameter with a
(void) cast in their telemetry-disabled arm. The attribute is this codebase's
idiom: it appears 105 times across 40 files on develop. Parameter names stay,
because the enabled arm uses them and the @param lines name them.

totalAgreementsEver() and totalMissedEver() are pure accessors and gain
[[nodiscard]]; no caller discards either.

include/xrpl/telemetry/Recording.h

@@ -174,12 +174,10 @@ public:
      * @param n  How much to add; defaults to 1.
      */
     void
-    add(T const n = 1) noexcept
+    add([[maybe_unused]] T const n = 1) noexcept
     {
 #ifdef XRPL_ENABLE_TELEMETRY
         value_.fetch_add(n, std::memory_order_relaxed);
-#else
-        (void)n;
 #endif
     }
 

include/xrpl/telemetry/Telemetry.h

@@ -336,10 +336,9 @@ public:
      * @param id  The node's base58-encoded public key.
      */
     virtual void
-    setNodeId(std::string const& id)
+    setNodeId([[maybe_unused]] std::string const& id)
     {
         // Default no-op for NullTelemetry implementations.
-        (void)id;
     }
 
     /**

src/xrpld/telemetry/ValidationTracker.h

@@ -226,7 +226,7 @@ public:
      * counter validation_agreements_total. See the counting-semantics
      * note in detail/ValidationTracker.cpp.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     totalAgreementsEver() const;
 
     /**
@@ -238,7 +238,7 @@ public:
      * counter validation_missed_total. See the counting-semantics note
      * in detail/ValidationTracker.cpp.
      */
-    uint64_t
+    [[nodiscard]] uint64_t
     totalMissedEver() const;
 
     /**